The optical storage and retrieval of information in holographic form has the potential to provide very high storage densities. Furthermore, by storing a given unit of information in a redundant fashion within a storage medium, there is provided a tolerance against a loss of information in any one given location within the storage medium. Also, a potential exists to store a large number of units of information at a given storage location, thereby further increasing storage density.
As can be appreciated, the realization of these various aspects of an optical information storage and retrieval system relies to a great degree upon the optical components and, in particular, upon the mechanisms for accurately scanning object and reference optical beams onto the storage medium. Typically, both beams are used simultaneously in order to store an information-encoded interference pattern into the storage medium. For information retrieval, only the reference beam is employed, in conjunction with a radiation detector array.
One conventional approach employs a rotating crystal to vary an angle of incidence of the reference optical beam upon the storage medium. This technique, known as angle multiplexing, enables a plurality of interference patterns to be stored within a region of the storage medium. The use of a rotating crystal implies that a mechanical assembly be employed. However, for a number of reasons the use of mechanical components is undesirable. For example, mechanical components generally require a significant amount of power to operate, occupy a significant amount of space, and may present both a repeatability and a reliability problem.
In an article entitled "Storage of 500 high-resolution holograms in a LiNbO.sub.3 crystal", Optics Letters, Vol 16, No. 8 (Apr. 15, 1991), F. H. Mok, M. C. Tackitt, and H. M. Stoll describe the recording (at room temperature) of as many as 500 high-resolution, uniformly diffracting volume holograms in a single Fe-doped LiNbO.sub.3 crystal. The holograms were stored by angularly multiplexing a plane-wave reference beam. The incidence angle of the reference beam was changed by using an optics assembly having a mirror mounted on a rotation stepper motor and a 1X telescope focussed at infinity.
It is an object of this invention to provide an optical information storage system that employs a fixed array of reflectors to spatially scan an angle multiplexed reference beam onto a holographic storage medium.
A further object of the invention is to provide a beam steering apparatus for positioning a reference optical beam at a prescribed location and with a prescribed angle of incidence upon a holographic storage medium.